This relates generally to image sensors, and more specifically, to image sensors with light guides.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include an array of image sensor pixels each of which includes a photodiode and other operational circuitry such as transistors formed in the substrate.
A dielectric stack is formed on the substrate over the photodiodes. The dielectric stack includes metal routing lines and metal vias formed in silicon oxide. Light guides are often formed in the dielectric stack to guide the trajectory of incoming light. A color filter array is formed over the dielectric stack to provide each pixel with sensitivity to a certain range of wavelengths. Microlenses are formed over the color filter array. Light enters the microlenses and travels through the color filters into the dielectric stack.
Ideally, light entering the dielectric stack is directed to corresponding photodiodes using the light guides. In practice, however, stray light can exit the light guides at odd angles, and regions on the substrate in between neighboring photodiodes may be inadvertently exposed to light. Regions on the substrate where light should not be allowed to enter may be referred to as “dark” regions. Pixel circuit structures such as polysilicon gate conductors and shallow trench isolation (STI) structures are usually formed in these interposing dark regions. Light hitting the silicon oxide/polysilicon interface and the STI/silicon substrate interface may be scattered in random directions, which results in elevated optical pixel crosstalk and a degraded pixel modulation transfer function (MTF).
It would therefore be desirable to be able to provide improved image sensors.